Elemental carbon is known to have many allotropes, such as diamond, graphite, the buckminsterfullerenes, and graphene. Graphene, in particular, has been receiving increasing attention for potential practical application of its special electronic and thermal transport properties. Graphene is one example of a carbon allotrope that is substantially characterized by a two-dimensional lattice of sp2-bonded carbon atoms. In our terminology, monolayers and bilayers of sp2-bonded carbon atoms are, respectively, “monolayer” and “bilayer” graphene. As further layers are added, the physical properties of these materials are known to progressively become less characteristic of graphene and more characteristic of bulk graphite. Accordingly, we refer to sp2-bonded carbon structures of multiple layers, generally five to ten layers, as “graphitic” carbon.
Numerous methods have been reported for preparing graphene films. These include cleavage of highly oriented pyrolytic graphite, reduction of graphite oxide, thermal decomposition of silicon carbide, chemical vapor deposition, and segregation using nickel and copper catalysts. In particular, a nickel catalyst has been used in conjunction with sacrificial carbon sources such as PDMS, PMMA, and polystyrene to produce bilayer graphene films, as reported, e.g., by Z. Yan et al., “Growth of Bilayer Graphene on Insulating Substrates,” ACS Nano 2011, 5, 8187-8192 and Z. Peng et al., “Direct Growth of Bilayer Graphene on SiO2 Substrates by Carbon Diffusion through Nickel,” ACS Nano 2011, 5, 8241-8247. In that process, a nickel coating encapsulates the carbon source, which is then annealed. The carbon atoms diffuse through the metal film to form the bilayer graphene film. Such a process, however, might be limited in its ability to produce complex structures of graphene, especially complex three-dimensional structures.
Fabrication of three-dimensional graphene structures using a template-directed chemical vapor deposition (CVD) process has been reported by Z. Chen et al., “Three-Dimensional Flexible and Conductive Interconnected Graphene Networks Grown by Chemical Vapour Deposition,” Nat. Mat. 2011, 10, 424-428. As reported there, graphene films are precipitated on a three dimensional scaffold of nickel foam, which is subsequently removed by etching. Such a process might also be limited in the kinds of structures that it can achieve in graphene.
Accordingly, there remains a need for further techniques for forming objects from sp2-bonded carbon, including graphitic carbon, and especially for forming three-dimensional objects from graphene films.